Electrocardiogram (ECG) – II

       Background Information

1. Practice on designing and building a non-inverting amplifier using an op-amp to amplify ECG waveform.

2. Experiment on filtering the ECG waveform with a low-pass filter.

In the previous lab, we have mentioned that signal analysis and signal processing are two important procedures in engineering practice. Now, let us apply these procedures to the task of measuring ECG waveforms.

The purpose of signal analysis is to understand the signal that we are measuring. For example, using signal analysis we may obtain the following list of the facts about the signal picked up by the electrodes placed on RA and LL in the ECG experiment. (1) The magnitude of the R-wave is about 1 – 2 mV. (2) The frequency range of the ECG signal is 0.1 - 250 Hz. (3) Besides the ECG waveform, the signal picked up by the electrodes also contains several kinds of noise: a low-frequency ( < 0.03 Hz) noise produced by respiration and electrode movement that results in a base line drift of the ECG signal, an EMG noise which has a wide frequency range (1 – 5000 Hz), and a 60 Hz noise of power line interference. The magnitudes of these noises are comparable to that of the ECG waveform.

Based on information gathered by signal analysis, we can design specific procedures for signal processing. (1) In order to enlarge the R-wave to about 0.5 – 1 V, the signal needs to be amplified by an amplifier (or several amplifiers) with a total gain of about 500. (2) To remove the low-frequency noise, a high-pass filter can be used. The corner frequency of the filter should be between 0.03 to 0.1 Hz. (3) Since the other two kinds of noise have frequency ranges that are overlapping with that of the ECG waveform, they are more difficulty to remove. The EMG noise can be reduced by requiring the subject to maintain motionless during the measurement. To reduce the 60 Hz noise, a special filter called a "notch filter" can be used. Since we have not learned the notch filter, we will try to use either a high-pass filter with a corner frequency above 60 Hz, or a low-pass filter with a corner frequency below 60 Hz, to reduce the 60 Hz noise. But which filter to use? We know that by using either filter, the ECG waveform will also be affected (distorted) because the ECG waveform contains useful information both in the frequency range above and below 60 Hz. The question then is: which filter will produce less distortion to the ECG waveform? Here we again need the help of signal analysis. More detailed signal analysis indicates that the P-wave and T-wave mainly contain frequency components that are far below 60 Hz. The R-wave also mainly contains frequency components that are below 60 Hz but it also contains some frequency components that are beyond 60 Hz. Therefore, we decide to use a low-pass filter with a corner frequency below 60 Hz to reduce the 60 Hz noise. Such a filter will: (1) effectively reduce 60 Hz noise, (2) have little effects on P-wave and T-wave, and (3) produce some distortion on the R-wave. In this lab, you will use a low-pass filter with different corner frequencies to filter the recorded signal and observe the effects of the filter on the signal.

The procedures of signal processing used in a particular application are commonly described by a block diagram. Following is a block diagram for the signal processing used in this lab.

Figure 1 – Block diagram of the signal processing used in this lab.

Notice that we have not specified the corner frequency of the low-pass filter.

The actual setup and circuit for this lab is shown below.

                Figure 2 – The setup for picking up and processing ECG signal.

In Fig. 2, different colors are used to help you to identify different stages in the overall signal processing. The circuit around AD620 with blue color corresponds to the block "1st Amplifier" in Fig. 1. The gain of the circuit is determined approximately by the following formula: G = 50 K/ R₁. The circuit with green color (C₁ and R₂) is the high-pass filter. The circuit around 741 with brown color is the 2nd Amplifier and the circuit with purple color (R₅ and C₂) is the low-pass filter.

Procedures

Due to the relative complexity of the circuit used in this lab, use fixed resistors and capacitors to build the circuit, except R₅ and C₂ which are decade resistance and capacitance boxes.

(1) Re-build the circuit used in ECG – I lab

                        Figure 3 – The circuit for the first amplifier and high-pass filter.

Figure 3 shows the same circuit that you have used before. The only change is the value of R₁: instead of 200, it is 1 KW now. As a result, the gain of the amplifier is reduced 5 times. After building the circuit, turn on the power supply and use the oscilloscope to verify that you get a valid ECG output. Write down the approximate magnitude of the R-wave.

Turn off the power supply now, but do not remove the electrodes.

(2) Add on the second amplifier (non-inverting amplifier)

Based on the value of R₄ determined in pre-lab preparation, add on the circuit for the non-inverting amplifier, as shown in Fig. 4. You may not be able to find a resistor for R₄ that has the exact value you have calculated. That is O.K. Just find a resistor of a closed value. After building the circuit, turn on the power supply and use the oscilloscope to verify that you get a valid ECG output. Write down the approximate magnitude of the R-wave, and compare with the R-wave magnitude that you obtained in step (1). What is the actual gain of your second amplifier? (find the ratio of the new R-wave magnitude and the old R-wave magnitude that you obtained in step (1))

                Figure 4 – The circuit for the first amplifier, high-pass filter and the second amplifier.

Turn off the power supply now, but do not remove the electrodes.

(3) Add on the low-pass filter

Now you should complete the circuit as shown in Fig. 2 by adding R₅ (using a resistance box) and C₂ (using a capacitance box). The value of C₂ is 1 m F and the initial value of R₅ is 1 K.

After building the circuit, turn on the power supply and use the oscilloscope to verify that you get a valid ECG output. If you are satisfied with the waveform on the screen, measure the approximate magnitude of the R-wave as well as the magnitude of the T-wave. Fill in these values in Table 1. Next, you are going change the value of R₅ to 2K, 5K, 10K, 20K, and 30K. You may first survey the effects of R₅ by flipping the switches on the resistance box to quickly change its value from 1 K all the way to 30 K. Then, for each R₅ value, observe the effect of the filter on the 60 Hz noise, and measure the magnitude of R-wave and T-wave and fill in Table 1. (The column of corner frequency should have already been filled during the pre-lab preparation.) Finally, calculate the ratio of the magnitude of R-wave over the magnitude of T-wave in each case and fill in the R-wave/T-wave column.

Table 1- Parameters of the low-pass filter (R₅ and C₂ = 1 m F), and the effects on the ECG signal

R5	Corner Frequency	Magnitude of R-wave	Magnitude of T-wave
1 K
2 K
5 K
10 K
20 K
30 K

Based on your observations, answer the following questions:

1. How low has the corner frequency to be in order for the filter to have effects on the 60 Hz noise?
2. When the corner frequency gradually decreases, does the low-pass filter become more effective in removing the noises?
3. When the corner frequency gradually decreases, how are the R-wave and T-wave affected by the filter?
4. If you are asked to design a definitive low-pass filter, what is your choice of the corner frequency? (the best filter should maximally reduce the noises while not cause significant distortion on the P-wave, R-wave and T-wave).

Pre-lab Assignment

1. Determine the gain of the first amplifier in Fig. 2.
2. Determine the corner frequency of the high-pass filter with the values of C₁ and R₂ indicated in Fig. 2.
3. Calculate the value of R₄ for the gain of the second amplifier (741) in Fig. 2 to be 10.
4. Calculate the corner frequency of the low-pass filter for each of the following values for R₅, and then fill in the "Corner Frequency" column of Table 1 later in this lab (note: C₁ = 1 m F).

R₅ = 1K, 2K, 5K, 10K, 20K, 30K